DE2540708A1 - DEVICE FOR DETERMINING THE STATUS OF FUEL RODS INSIDE A NUCLEAR REACTOR CORE - Google Patents

Description

PATENT APPLICATION Applicant; The Babcock & Wilcox Company

161 East 42nd Street, New York, N.Y. 10017 - USA -

Title: Device for determining the condition of fuel rods within a nuclear reactor core

The invention relates to a method and a device for inspecting and determining leaky fuel rods within a nuclear reactor core.

Nuclear reactors for power generation, research and other purposes require a "critical ¹¹ concentration of fissile material. This concentration - in the reactor core - often comprises a bundle of long, slender, hollow metal rods or" sleeves ", each of which contains a stack of fuel sintered bodies The most common fuel sintered bodies are made of uranium dioxide (UO2) ·

During the operation of the reactor, some of the fissile uranium nuclei in these fuel sintered bodies absorb neutrons, which

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Causes the uranium nuclei that have been hit to split up into two new nuclei fissures, each with about half the atomic mass of the uranium parent nucleus. These new nuclei, which are often radioactive, should be inside of the surrounding metal rod. The escape of these radioactive materials from the fuel rod structure could create serious safety problems lead or at least to the need to carry out a complex and costly radiation decontamination.

The environmental conditions in power reactors tend to make this situation even more difficult. So can high temperatures, high Pressures and intense radiation inside the core cause occasional fuel rod damage in the form of a tiny broken case cause.

In a pressurized water reactor, for. B. the "primary coolant" Pressurized water that removes heat from the reactor core is monitored for radioactivity. A noticeable increase in radioactivity this water above a predetermined level is said to have damaged one or more fuel rods within the keactor core are. This conclusion is based on the assumption that these additional radioactive pollutants are likely through A leak has escaped from a fuel rod and has penetrated the neighboring water flow.

If the radioactive contamination of the primary coolant is determined to be undesirably high, then the reactor must be shut down and the individual fuel rods are inspected to determine the individual damaged rod or rods and replace. The task of inspecting these rods is very difficult and quite expensive, especially in a power reactor. So must z. B. opened the heavy reactor pressure vessel

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while each of the bundles of 200 or more fuel rods that the individual fuel assemblies from which the reactor core is assembled, removed from the core and individually checked for presence a damaged fuel rod must be inspected. In this regard, it should be noted that commercial power reactors could have up to 36,000 fuel rods or even more exhibit.

uas usually used inspection procedures referred to as "sipping", including the fuel assembly in a

sipping sleeve "broke in and haters or air through that fuel element pumped. The medium flowing out of the tlement becomes monitored for the presence of radiation. A finding that there is an excessive amount of spray in the drain indicates that either a fuel rod within the viewed fuel assembly may be damaged or that the medium from the rods is radioactive rinses that adhered to the stick surfaces. In addition to this ambiguous display, there is the further possibility that a rod that leaks under normal reactor operating conditions, perhaps under the conditions existing in the sipping sleeve is not leaking.

Another method requires the removal of a single fuel rod from a fuel assembly. This is of course a difficult and time consuming task, especially when the fuel in question is has been partially consumed or "burned" and generates so strong radiation that the fuel rod dismantling behind a Radiation shielding must be carried out with remote controlled manipulators.

The rod in question is closed in a container that has a chamber or filling chamber to collect fission products that

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may have escaped from the rod. Becomes a radiation monitor held at a distance from part of the filling chamber by a radiation-absorbing screen, which is a channel or collimator which aligns the radiation monitor with the part of the filling chamber that is under observation. Generated this way the guard sends an electrical signal depending on the radioactive fission products that may have escaped from the fuel rod are. Of course, an extremely high radiation in the filling chamber part indicates that there may be a break in the fuel rod. This method is also based on the monitoring of fission products outside the fuel rod sleeve. Namely, know the fuel rod in question is leaking during reactor operation, but not within the inspection atmosphere, then a fuel rod could be damaged but not be detected as such.

Regarding the monitoring of these radioactive fission products is determine that a number of radioactive elements are emitting gamma rays. In many cases these gamma rays have energies which apply to a specific isotope of the element concerned,

133 ζ-. B. Xe, or it sends the isotopes of the element xenon, the one Has atomic mass of 133 units, a characteristic gamma ray of 0.05 million electron volts (MeV) of energy. The "half-life" this isotope is also about 5.3 days. Thus, during half of that in a given period of 5.3 days

133 Amount of Xe atoms present a gamma ray of 0.085 MeV each have broadcast. Since the gamma ray emissions from radioactive Cores are essentially arbitrary in nature, it is used to unambiguously determine the presence and concentration of one certain substance requires that the guard observe a sample for a sufficient period of time to be statistically meaningful To collect the number of detected gamma rays from an energy which is characteristic for the substance in question. These

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statistical determination depends on a number of quantities: Sensitivity of the guard, level of background radiation and the like. However, this is a problem that is common to those skilled in the art of nuclear instruments successfully for many years.

As a result, a radiation monitor is required for this type of work, which responds to the energy of the incident gamma radiation. In general, crystal scintillation detectors and Suitable for semiconductor detectors. In particular, a semiconductor detector, typically one of "lithium permeated." Germanium crystal "or part of a crystal, to an incident gamma ray by generating an electrical signal that is related to the energy lost by the gamma ray within the crystal structure. That created in this way electrical signal is processed to determine these incident radiations. In this context, a conventional separates electrical system including amplifiers and pulse height discriminating circuitry from those signals which Determine gamma rays in the energy range which is of interest is. Each of these detected signals is called a "count" and is added to a cumulative sum of these counts, to get an indication that the isotopes in question is present and, under appropriate circumstances, a further display of the concentration of isotopes in the relevant Sample.

Thus, the prior art has offered some means of eliminating defective Monitor and determine fuel rods. However, these prior art methods are time consuming, ineffective and often lead to ambiguous or unreliable results. As a result, there is a need for an improved method

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for the detection of damaged tennis bars, especially within the reactor tank.

The invention is therefore based on the object of a device to create that makes it possible to quickly and easily determine the condition of fuel rods within a reactor core.

This object is achieved according to the invention in that a radiation energy-sensitive Guardian is enclosed in a shielded container made of iron or equivalent material consists. The screen is also provided with a small collimating "tunnel" through which the radiation coming from only one Direction that Guardian can reach. The collimator is aligned with and in the top of one of the fuel rods Alignment held for a sufficient amount of time to allow that a statistically meaningful number of counts is collected to determine the presence of an appropriate concentration of radioactive Detecting fission products inside the fuel rod sleeve, they know corresponding concentration of fission products or a specific fission product within the sleeve is determined by the monitor, it can be assumed that the fuel rod is undamaged. Should the observed fission product inventory within the fuel rod be lower as a set point corresponding to the use and irradiation of the rod concerned, it can be assumed that the rod concerned The rod is broken and some of the fission products have escaped.

According to the invention, the shielded and collimated guard observes the fission product concentration within the fuel rod, thereby providing an indication of the undamaged condition of the fuel rod regardless of the staff's ability to divert fission products into a test atmosphere during the inspection.

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Furthermore, in this context, the screened and collimated Guardians can be mounted on a switching device, optionally the collimator and the guardian with the ends on top of each other aligns the following fuel rods that form the fuel assemblies in a reactor core; this can also be done in a fuel transport and -Jflgerkanal take place. This way the undamaged Condition of each fuel rod can be determined by direct observation without removing the individual rods from the reactor core or removed from the fuel bundle.

An exemplary embodiment of the invention is shown in the drawing and is described in more detail below. It shows:

Fig. 1 is a perspective view of a device for Application of the invention, and

Figure 2 is a perspective view of a schematic illustrating typical guard collimator fuel rod orientations.

As shown in Figure 1, a fuel assembly 10 includes a keihe parallel, spaced apart fuel rods 11, which are in the correct relative position to each other through transverse grids 12 being held. Each fuel rod 11 has a thin, metallic one Outer sleeve, which is a vertical column of sintered nuclear fuel bodies and an empty barely or enclosing a filling chamber, which is used for fuel rod pressure build-up and the reception of Fission gases is used.

The fuel assembly 10 ends in an upper and lower end piece 13 or 14, These end pieces fulfill several related tasks with the fuel rod stabilization, the installation of the fuel assembly

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in the reactor core, the stability of the fuel element in the reactor core, the removal of the fuel assembly from the reactor core, etc. For the invention, however, it is only necessary that the insert 15 of the upper end piece of a generally open frame construction exists, which exposes or does not darken, cover or shield the transverse ends of the individual fuel rods.

To explain the invention, the fuel assembly 10 is shown individually in FIG. In practice, however, the invention, described in greater detail below, can be practiced with the fuel assembly after use in a reactor core and in a highly radioactive state. In order to protect the personnel from the physically harmful effects of this radiation, the fuel assembly 10 must be shielded accordingly. So z. B. in a pressurized water reactor water with a depth of 3 meters between the fuel assembly 10 and the plant personnel form a suitable radiation shield. Furthermore, in this regard, the individual rods 11 within the fuel assembly 10 could also be inspected in accordance with the principles of the invention, while the fuel assembly is part of the larger fuel assembly that constitutes the reactor core.

Above the upper end piece 13 and in general alignment too A generally cylindrical radiation shield 16 is disposed on the fuel assembly 10.

The vertical axis of the cylindrical screen 16 (not in Fig. 1 shown) is in alignment with a narrow collimator 17 which forms a channel which of the one close to the upper one End piece 13 lying transverse end 20 of the screen 16 starts, and within of the screen has a substantially central cavity 21,

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which an energy-sensitive radiation monitor 22 with high Resolving power absorbs; A typical guard of this type is the germonium semiconductor guard traversed by lithium. Farther In this regard, it is often necessary to cool monitors of this type to extremely low temperatures. To this cooling to make possible, the! eighth is often in a specially shaped Dewar bottle arranged containing a supply of liquefied Contains nitrogen or the like. Thus, not only is the guard 22 arranged in the screen 16, but the screen contains it also cooling devices (not shown), as well as corresponding electrical switching connections for the guard (also not shown).

The screen 16 and the associated radiation monitor are located in Alignment with each of the individual fuel rods 11 in the fuel assembly 10 by a switching device 23, which is shown schematically in Figure 1 as a pair of vertically arranged arrows is shown. For explanation it should be noted that the switching device the Screen 16 in two directions above the fuel rods 11 in the fuel assembly 10 can implement in order to arrange it correctly, either manually depending on an optical alignment or automatically in accordance with predetermined fuel rod positions within the fuel assembly with respect to an or multiple set marks. Reference is also made to Figure 2 which shows a portion of the top of some of the fuel rods show in a fuel assembly 10, which was explained in Figure 1. The upper end piece 13, which is also shown in Figure 1, is shown in FIG Figure 2 is not shown in order to clarify the collimator alignment methods described in greater detail below. Accordingly, the fuel rods 11 have fuel pellets 24, which are in thin tubes 25 made of aluminum, non-

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stainless steel, zirconium or the like are included. Each This tube 25 is also closed by means of corresponding Enakaps 26. As shown in the drawing, the stacks of fuel pellets 24 do not extend over the entire interior the closed tubes 25, but end at a certain distance below the end caps to form a filling chamber 27, absorbs the fission product gases and the like. Xenon and especially

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The radioactive Xe - isotopes are a typical fission product gas, which has a tendency to collect in the filling chamber 27.

If, in the application of the invention, the xenon isotopes that characterize these Gamma ray 0.085 MeV is chosen to examine the fuel rod for damage, then the collimator 17 (Figure 1) in the radiation shield 16 can be arranged in a vertical alignment to the longitudinal axis of one of the tubes 25, as shown in FIG is. With this arrangement, the guard is only responsive to those gamma rays that are in general alignment with the guard are directed through the filling chamber 27 with and within a projection 30 of the collimator. Those belonging to the guardian Radiant energy discrimination circuitry rejects incident radiation signals from the monitor that are not the observed ones Gamma rays correspond to 0.085 MeV. Those signals which are passed through the discrimination circuitry are recorded to the cumulative "counts" of the respective To yield radiant energy. This cumulative "count" relates

133
of course on the Xe concentration within the filling

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chamber 27. The time during which the Xe "counts" are to be summed depends, of course, on the specific "counting statistics" that characterize the size of the fuel rods to be inspected. As previously mentioned, these statistical determinations are based on one

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to a certain extent on the guard's sensitivity, the background beam

133 ment and the like, as well as the presumed Xe concentration within a given fuel rod. So if a predetermined

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If the number of cumulative Xe counts does not occur from a single training rod within a given period of time, then one can use

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Correctly conclude from this that the Xe gas concentration within the filling chamber 27 is below the level that is to be expected from an undamaged fuel rod . Under these circumstances, the fuel rod under test is likely to be leaking

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Location that allows the Xe to flow off the fuel rod instead of collecting in the fill chamber 27.

On the other hand, when the accumulated counts reach the target value or exceed, then it can be assumed that fission products have not emerged from the brain rod and that the rod is undamaged is. The invention thus makes it possible to determine the undamaged state of the fuel rod by direct measurement of the fission product supply to be determined within the staff in contrast to the inaccuracies of previous methods, which in an examination of the fission products passed after leaving the inside of the rod.

It is pointed out in FIG. 2 that the collimator projection 30 is in direct alignment with the longitudinal axis of the fuel sintered body stack 24. Special circumstances, e.g. B. the dwell time of the fuel rod in the nuclear reactor core, can generate an extremely high intensity of the background radiation from the fuel sintered body stack 24. When the collimator with the longitudinal axes of the fuel sintered body stack 24 and the associated tube 25 is in alignment, as shown in connection with the Kollimatorvorsprung 30, the high-intensity background radiation of the sintered bodies may tend to be located in observation less

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to darken intense Xe radiation. Under these circumstances

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it is advisable to place the collimator 17 (Figure 1) in the radiation shield 16 (also Figure 1) to bend to a collimator projection 31 (Fig. 2) to produce, which forms an acute kink with the longitudinal axis of the fuel rod under test. The right Choice of the angle should enable the collimator projection 31 to penetrate the filling chamber 27 completely and yet to avoid a collision with a substantial part of the pellet stack 24. As shown in the drawing, one falls further continuation of the collimator projection 31 beyond the surface of the tube 25 with the pellet stack in at least one of the adjacent pipes together. In most cases, however, the space between the adjacent fuel rods is filled with water or a filled with another suitable substance, which serves to shield or absorb those radiations from the neighboring ones Run out of fuel rods which are in general alignment with the bent collimator projection 31. This absorption tends of course to weaken the influence of the direct sintered body radiation on the radiation background, which in the guard 22 (Figure 1) occurs, and thereby enables the

133 Xe radiation easier to distinguish.

In operation, the radiation screen 16 (Figure 1) above the Upper end piece 13 arranged in alignment with the longitudinal axis of one of the fuel rods 11. This orientation can be either used for Explanation in Figure 2 of the drawing correspond to collimator arrangements or another orientation that is for a a certain fuel rod design, a certain reactor core environment or other considerations is more suitable, depending on the Dictate circumstances.

The gamma rays that characterize the radioisotopes which are for

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the observation has been selected (e.g. Xe) will penetrate the collimator and are registered by the guard 22 to accumulate in a "counter" or the like for a period of time determined primarily by statistical considerations will. After an acceptable cumulative count is made, can a determination of the undamaged condition of the fuel rod can be made. Typically, a count states that is significantly below the "normal" count (or count rate if counts per unit of time are selected as the basis for radioisotope observation that a fuel rod is damaged. "Normal" response from the guard or a stronger than "normal" response from the guard states, however, that the fuel rod under test is structurally in perfect condition because it appears that none of the fission product stocks escaped from the rod.

The principles of this invention can find utility in a number of structural arrangements other than those described above differ. In this context, a probe-mounted guard of appropriate dimensions could be placed between the fuel rods be introduced to monitor the inventory of fission products within the fuel rods.

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Claims (3)

PATENT CLAIMS: 1J device for inspecting the state of radioactive fuel rods, characterized by a radiation _{screen (16) f} which has a radiation collimator (17) formed therein, a radiation energy-sensitive monitor (22) within the screen (16) and in alignment with the collimator (17) for Generating a signal _{p identifying} at least one radioactive material within the fuel rod (11) and means for aligning the collimator and monitor with the fuel rod. 2. Device according to claim 1, characterized in that that the radiant energy-sensitive guard additionally comprises a lithium-permeated germanium semiconductor guard. 3. Procedure for inspecting the condition of radioactive fuel rods, characterized by the following process steps: a: align a radiant energy sensitive guard with one of the fuel rods and b: count the radiations emanating from the rod, which are at least mark one of the radioactive materials within the rod, to investigate,
provided that Pob radioactive material within the staff has been retained. 809838/0211 Blank page